The conversion of complex organic matter to methane is an important component of the global carbon cycle. The process occurs in a variety of anaerobic environments; for example: (i) the lower intestine of humans, (ii) sewage treatment plants, and (iii) the sediments of lakes, streams and rivers. Thus, biological methanogenesis impacts the environment and human health in several ways. For example, methane is a major "greenhouse" gas. on the other hand, the process of methanogenesis is used to treat industrial and domestic wastes including toxic compounds. Methanogenic organisms are archaebacteria which are different from eubacteria and eukaryotes at the most elemental level. Thus, an understanding of these unusual organisms requires a study of fundamental cellular processes. The current understanding of methanogenic archaebacteria is largely at the level of microbiology and biochemistry; less is known concerning the regulation of gene expression. Here, we propose a molecular genetic approach to investigate gene expression in the pathway of acetate conversion to methane which is tightly regulated in response to the growth substrate. The results are expected to extend the current view of methanogenesis to include principles of gene expression. The results are also expected to compliment on-going biochemical studies on the pathway in Methanosarcina thermophila. The long-term goal is to utilize in vivo molecular approaches to study (i) fundamental principles of transcription in the archaebacteria, and (ii) the mechanism of enzymes in the acetate-to-methane pathway. The research proposed here, together with parallel research to characterize mutants and develop a transformation system, will lay the foundation for these long-term goals.